Germanium-containing particulate capture from an exhaust gas stream of a glass production system

ABSTRACT

The invention relates to a method and apparatus for a glass manufacturing facility. The apparatus includes a glass production system providing an exhaust stream entrained with particulate material including germanium—containing particulate. The apparatus further includes an exhaust filtration system including a PTFE fabric material. The exhaust filtration system is connected to the production system to receive the exhaust stream an capture the particulate material. A collection system is connected to the exhaust system to collect the captured particulate material from the exhaust system. The particulate material collected contains at least about 2% by weight germanium. The invention further includes incorporating the aforementioned apparatus into the operation of a glass manufacturing facility.

This application is a 371 of PCT/U.S. 98/27779 filed Dec. 29, 1998,which claims benefit of provisional application No. 60/070,235, filedDec. 31, 1997.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

This invention relates to production of germanium-containing glass. Theinvention is more particularly concerned with an improved glassmanufacturing facility and method of operation, whereingermanium-containing particulate filtered from an exhaust gas stream ofthe glass production process is collected from a filtration system at aconcentration level conducive to reclamation of the germanium.

BACKGROUND OF THE INVENTION

In the manufacture of germanium-containing glass for various advancedoptical products, such as optical fibers, germanium dioxide (GeO₂)particulate, silicon dioxide (SiO₂) particulate, hydrochloric acid (HCl)fumes, and water vapor are produced as byproducts. These byproducts areexhausted to a pollution abatement system where, among other things, theGeO₂ and SiO₂ particulate are captured from the exhaust stream andcollected for disposal in a landfill.

FIG. 1 is a basic block diagram of a conventional glass manufacturingfacility 10 that produces germanium-containing glass. The facilitycomprises a glass

production system 12 including a plurality of lathes 14. The lathesproduce bodies of GeO₂-doped silica glass, commonly called blanks orpreforms, using a process known as flame hydrolysis. The aforementionedbyproducts are reaction products of this process.

In the lathes 14, vaporous SiCl₄ and GeCl₄ as raw materials are passedthough a specially designed methane burner in precisely controlledamounts depending upon the desired constitution of the silica glassblanks. The SiCl₄ and GeCl₄ are reacted with oxygen under the heat ofthe burner to form minute particles or “soot” of SiO₂ and GeO₂. Aportion of these particles is deposited on the outer periphery of arotating mandrel (a technique known as outside vapor deposition or OVD)to form the glass blanks. The excess soot is exhausted from the lathes.

The respective exhausts of the lathes 14 are connected to a dedicatedloop L that pulls air from the lathes by way of a loop exhaust fan 16.The fan circulates the exhaust stream to a baghouse 18, including aplurality of baghouse modules 20 (four in the form shown), forfiltration of the SiO₂ and GeO₂ particulates from the stream. Afterfiltration by the baghouse 18, a portion of the exhaust stream isrecirculated through the loop and mixed with pre-heated makeup air. Thenon-recirculated portion of the exhaust stream is supplied, via ascrubber fan 22, to a scrubber system 24 and then discharged through anexhaust stack 26. The scrubber system 24 scrubs the filtered exhauststream from the baghouse with weak acid or soft water to removevapor-phase chloride components, including HCl, SiCl₄ and GeCl₄, toensure that the discharge from the exhaust stack complies withenvironmental requirements.

In conventional operation of the production system 12, different typesof blanks are run on individual lathes depending upon customer demand.At any given time, some of the lathes 14 may be running a lowconcentration of germanium, while others may be running a highconcentration of germanium, with still others running at intermediateconcentration. GeO₂ content may differ between individual blanks by asmuch as about 13-14% by weight, and possibly more. It will thus beappreciated that the GeO₂ concentration in the exhaust stream suppliedto the baghouse system varies widely, depending upon the particular mixof blanks being manufactured a given time.

It should be noted, incidentally, that the earlier-described productionprocess of the lathes 14 is merely exemplary. As is well known to thoseskilled in the art, the flame hydrolysis process can be implementedusing materials other than those mentioned above. And soot depositioncan be accomplished by other techniques such as modified chemical vapordeposition (MCVD), in which soot is deposited on the inner periphery ofa rotating hollow mandrel, and vapor axial deposition (VAD), in whichsoot is deposited on the axial end of a rotating rod. Indeed, as shownin FIG. 1, the glass production system 12 includes additional lathes 28that produce blanks of germanium-free silica glass, and these lathes arealso connected to the loop L.

The SiO₂ particulate and the GeO₂ particulate byproducts from the lathes14 are generally in the size ranges of 0.5-1 μm and 0.05-0.5 μm,respectively. To capture these particulates, as well as the particulatesfrom the lathes 28, the baghouse modules 20 use acrylic filter bags 21that are pre-coated (pre-loaded) with particulate SiO₂. The exhauststream from the lathes is passed through the pre-coated bags which trapand accumulate substantially all of the particulate material entrainedin the exhaust stream. Each of the baghouse modules is periodicallytaken off-line and isolated from the loop L in order to conduct acleaning cycle. During the cleaning cycle, a mechanical shaking system(not shown) shakes the bags of the isolated module to dislodge theaccumulated particulate material captured from the exhaust stream. Thecleaned bags are then recoated with SiO₂ from a supply system 30connected to the baghouse, and thereafter placed back on-line.

The material dislodged from the filter bags during the cleaning cycle iscollected and landfilled in an appropriate facility. By landfilling theparticulate mixture collected from the baghouse, there is substantialwaste of costly germanium.

Technology exists and is commercially available for reclaiming germaniumfrom particulate GeO₂. But, in the conventional facility usingpre-coated bags and operated as described above, the concentration levelof germanium in the material collected from the baghouse is insufficientfor cost-effective reclamation. As a practical matter, a minimumconcentration of 2% germanium by weight is ordinarily required.

SUMMARY OF THE INVENTION

In accordance with the present invention, it has been discovered that byappropriate adaptation of the manufacturing facility design andoperation, it is possible to collect a substantial portion of theheretofore wasted germanium in sufficient concentration to permitcost-effective reclamation. In particular, it has been discovered thatby modifying the baghouse to use filtration bags having a PTFE(polytetrafluoroethylene) filter membrane supported on a PTFE backingfabric, and by dedicating the glass production system connected to thebaghouse to the production of blanks having sufficient germaniumconcentration, it is possible to obtain a particulate mixture from thebaghouse that meets or exceeds the aforementioned 2% level. For example,in a preferred mode of the invention to described in detail later, theproduction system is constituted by lathes which are dedicatedpredominantly to the production of blanks for use in making multi-modeoptical fibers.

Briefly stated, in accordance with one of its broader aspects, theinvention provides a glass manufacturing facility which comprises aglass production system providing an exhaust stream entrained withparticulate material including germanium-containing particulate, and anexhaust filtration system including PTFE membrane filter materialsupported by PTFE fabric material, the exhaust filtration system beingconnected to the glass production system to receive the exhaust streamand capture the particulate material. The facility additionallycomprises a collection system connected to the exhaust filtration systemto collect the captured particulate material from the exhaust filtrationsystem. The glass production system operates to produce glass selectedsuch that the concentration of germanium in the particulate materialcollected by the collection system is at least about 2% by weight.

In a related aspect, the invention provides an operation method of aglass manufacturing facility, which comprises producing glass with aglass production system that provides an exhaust stream entrained withparticulate material including germanium-containing particulate,filtering the exhaust stream with an exhaust filtering system includingPTFE membrane filter material supported by PTFE fabric material, therebycapturing the particulate material, and collecting the capturedparticulate material from the exhaust filtration system with acollection system. As previously stated, the glass is selected such thatthe concentration of germanium in the particulate material collected bythe collection system is at least about 2 1% by weight.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned and additional aspects of the invention will bebetter understood from the following detailed description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram showing of a conventional glass manufacturingfacility for germanium-containing glass; and

FIG. 2 is a block diagram of a glass manufacturing facility according tothe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 diagrammatically depicts a glass manufacturing facility 10′ inaccordance with the present invention. The basic layout of this facilityis similar to that of the facility 10 shown in FIG. 1, except that noSiO₂ supply system is provided for the baghouse 18′. Beyond the absenceof this system, the principal differences from the conventional facilityrelate to the baghouse 18′ and the glass production system 12′. Thesedifferences are addressed below. As will also be explained, theparticulate collection system for the baghouse is designed to packagethe particulate in appropriate form for shipment to the germaniumreclamation service provider.

Baghouse 18′ in the facility of FIG. 2 is a modification of theconventional baghouse previously described, in which the standardpre-coated acrylic filtration bags are replaced with bags 21′constructed from a PTFE filtration laminate. The laminate materialcomprises, more particularly, an expanded PTFE filtration membrane(e.g., 0.01 inch thickness) bonded to a PTFE backing fabric thatsupports the membrane. The laminate material is commercially availableunder the trademark GORETEX from W. L. Gore & Associates, Inc., Elkton,Maryland. This material has been found to exhibit excellent resistanceto the high-temperature, acidic environment in the loop L′, and tocapture soot particles, such as SiO₂ and GeO₂ in the earlier-mentionedsize ranges with satisfactory efficiency.

Optimum cleaning of the PTFE filtration laminate bags may require thatincreased cleaning energy be imparted to the bags during the cleaningcycle. This can be accomplished by adjusting the shake amplitude andfrequency imparted to the bags by the mechanical shake systemincorporated in the baghouse modules. Sonic horns may also be installedin the baghouse modules to increase the cleaning energy to the bags.

Because the PTFE laminate bags do not require SiO₂ pre-coating, itbecomes possible to substantially reduce the concentration of SiO₂ inthe particulate material collected from the baghouse, thus effectivelyincreasing the concentration of GeO₂, and therefore, germanium. Tofurther increase the concentration of GeO₂ in the collected material,the glass production system 12′ is modified from the conventional systemsuch that preferably all of the lathes are dedicated to the productionof GeO₂-doped silica glass blanks.

The glass (or glasses) produced on the lathes 14′ is (are) preferablyselected such that the resulting exhaust stream from the lathes willprovide a sufficient amount of GeO₂ such that the germaniumconcentration in the material collected from the baghouse is at least 2%in order to allow for reclamation of the germanium. A sightly lowerconcentration may be acceptable in some applications, however. The GeO₂content of the exhaust stream from the lathes may vary, provided thatthe average content over a suitable period of time, such as the intervalbetween cleaning cycles, yields the desired concentration of germaniumin the particulate collected from the baghouse.

Another modification related to the baghouse involves the so-calledair-to-cloth ratio, which is defined as the ratio of nominal volumetricair flow through the baghouse in actual cubic feet per minute to thetotal on-line filter cloth area of the baghouse in square feet. It hasbeen determined that the PTFE laminate bags do not tolerate as high anair-to-cloth ratio as the conventional pre-coated filter bags in theenvironment of the invention. This factor should be taken intoconsideration in the overall facility design, because when an individualbaghouse module is taken off-line during a cleaning cycle or a serviceoutage, for example, the air-to-cloth ratio of the modules remaining inoperation will be increased correspondingly. In other words, theremaining modules bear the load of the module taken off-line.

Generally speaking, the PTFE laminate bags should not be subjected tosustained operation at an air-to-cloth ratio exceeding 1.1. Sustainedoperation above this ratio may cause excessive wear of the PTFE membraneand/or excessive caking of particulate on the bag material such that itcannot be adequately cleaned.

The air-to-cloth ratio for normal operation (all modules on-line) istherefore preferably set such that when the modules are individuallyoff-line, the air-to-cloth ratio of the remaining modules does notexceed 1.1. More preferably, the setting is such that the ratio of theremaining modules does not exceed 0.9. The capacity of the baghouseand/or the number of lathes connected to the baghouse may be determinedas needed to attain an appropriate ratio.

Returning now to FIG. 2, the baghouse particulate collection systemincludes a silo 32 which is connected to hoppers at the respective basesof the baghouse modules via a pneumatic transport system 34. Thepneumatic transport system transports particulate material dislodgedfrom the PTFE filter bags during the cleaning cycle for collection inthe silo. The silo feeds a mixer 36, where the collected particulatematerial is mixed with water from a water spray to compact it, whereuponit is dropped into bulk bags for shipment to the germanium reclamationservice provider (for reclamation in the form of GeCl₄). The water sprayfurnishes only such an amount of water that is sufficient to compact theparticulate without forming hard nodules, which are undesirable forreclamation. The compacted material thus remains substantially dry.

EXAMPLE

A glass manufacturing facility as shown in FIG. 2 was constructed andoperated in accordance with the following parameters:

Lathes: 6 lathes operated to produce GeO₂-doped silica glass blanks witha predominance of blanks for multi-mode optical fiber and a balance ofblanks for dispersion-shifted optical fiber

Baghouse Construction:

Number of baghouse modules—4 (same construction)

Bag material—GORETEX PTFE laminate (0.01 in. membrane thickness)

Bag size—5 in. diameter, 14 feet long

Cleaning Cycle—Every 10-18 hours depending on production conditions(total shake time 7 minutes per module)

Total bag (cloth) area—20,888 sq. feet

Flowing Temp./Baghouse Inlet Temp.: 190° F.

Loop Nominal Volumetric Flow Rate (@ 190° F.): 10,500 scfm; 12,877 acfm

Permissible Loop Temp. Range: 140-250° F. (dew point @ 130°F.)

Range of Air-to-Cloth Ratio:

All modules on-line—0.57 @ 140° F. to 0.67 @ 250°F.

One module off-line—0.76 @ 140° F. to 0.90 @ 250°F.

GeO₂ Concentration of Collected Particulate at Mixer Output (per 400 lb.particulate batch, dry weight basis): 2.7-6.7% by wt. depending on glassproduction mix

It is to be understood, of course, that the preferred modes of theinvention described above are merely exemplary and that variousimplementations are possible in keeping with the scope of the inventionas defined in the appended claims.

We claim as our invention:
 1. A glass manufacturing facility,comprising: a glass production system providing an exhaust streamentrained with particulate material includinggermanium-containing-particulate; an exhaust filtration system includingPTFE membrane filter material supported by PTFE fabric material, saidexhaust filtration system being connected to said glass productionsystem to receive said exhaust stream and capture said particulatematerial; and a collection system connected to said exhaust filtrationsystem to collect the captured particulate material from said exhaustfiltration system, said glass production system operating to produceglass selected such that the concentration of germanium in theparticulate material collected by said collection system is at leastabout 2% by weight.
 2. A glass manufacturing facility according to claim1, wherein said germanium-containing particulate is GeO₂.
 3. A glassmanufacturing facility according to claim 2, wherein the GeO₂ particlesize is in a range from 0.05-0.5 μm.
 4. A glass manufacturing facilityaccording to claim 1, wherein the glass produced by said glassproduction system is GeO₂-doped silica glass and said particulatematerial includes SiO₂ and GeO₂.
 5. A glass manufacturing facilityaccording to claim 4, wherein the GeO₂ particle size is in a range from0.05-0.5 μm and the SiO₂ particle size is in a range from 0.5-1 μm.
 6. Aglass manufacturing facility according to claim 5, wherein said glassproduction system includes a plurality of lathes that produce blanks forthe production of optical fiber.
 7. A glass manufacturing facilityaccording to claim 1, wherein said exhaust filtration system is abaghouse system including a plurality of baghouse modules using filterbags of a construction in which a PTFE filter membrane is supported on aPTFE fabric backing, and the air-to-cloth ratio of said baghouse systemis set such that when said modules are individually off-line, theair-to-cloth ratio of the remaining modules is less than or equal to1.1.
 8. A glass manufacturing facility according to claim 7, wherein theair-to-cloth ratio of said baghouse system is set such that when saidmodules are individually off-line, the air-to-cloth ratio of theremaining modules is less than or equal to 0.9.
 9. A glass manufacturingfacility according to claim 7, wherein said glass production systemincludes a plurality of lathes that produce GeO₂-doped silica glassblanks for the production of optical fibers, and the particulatematerial includes SiO₂ and GeO₂.
 10. A glass manufacturing facilityaccording to claim 9, wherein the Geo₂ particle size is in a range from0.05-0.5 μm and the SiO₂ particle size is in a range from 0.5-1 μm. 11.A glass manufacturing system according to claim 1 wherein saidcollection system comprises a mechanical shaking system.
 12. Anoperation method of a glass manufacturing facility, said methodcomprising: producing glass with a glass production system that providesan exhaust stream entrained with particulate material includinggermanium-containing particulate; filtering the exhaust stream with anexhaust filtering system including PTFE membrane filter materialsupported by PTFE fabric material, thereby capturing said particulatematerial; and collecting the captured particulate material from saidexhaust filtration system with a collection system, wherein the glass isselected such that the concentration of germanium in the particulatematerial collected by said collection system is at least about 2% byweight.
 13. A method according to claim 12, wherein saidgermanium-containing particulate is GeO₂.
 14. A method according toclaim 13, wherein the GeO₂ particle size is in a range from 0.05-0.5 μm.15. A method according to claim 13, wherein the glass produced by saidglass production system is GeO₂-doped silica glass and said particulatematerial includes SiO₂ and GeO₂.
 16. A method according to claim 15,wherein the GeO₂ particle size is in a range from 0.05-0.5 μm and theSiO₂ particle size is in a range from 0.5-1 μm.
 17. A method accordingto claim 16, wherein said producing includes operating a plurality oflathes to produce GeO₂-doped silica glass blanks for the production ofoptical fiber.
 18. A method according to claim 12, wherein said exhaustfiltration system is a baghouse system including a plurality of baghousemodules using filter bags of a construction in which a PTFE filtermembrane is supported on a PTFE fabric backing, and said filteringincludes setting the air-to-cloth ratio of said baghouse system suchthat when said modules are individually off-line, the air-to-cloth ratioof the remaining modules is less than or equal to 1.1.
 19. A methodaccording to claim 18, wherein the air-to-cloth ratio of said baghousesystem is set such that when said modules are individually off-line, theair-to-cloth ratio of the remaining modules is less than or equal to0.9.
 20. A method according to claim 18, wherein said producing includesoperating a plurality of lathes to produce GeO₂-doped silica glassblanks for the production of optical fiber, and the particulate materialincludes SiO₂ and GeO₂.
 21. A method according to claim 20, wherein theGeO₂ particle size is in a range from 0.05-0.5 μm and the SiO₂ particlesize is in a range from 0.5-1 μm.
 22. A method according to claim 12wherein said collecting comprises a mechanical shaking system.
 23. Amethod according to claim 12 wherein said collecting comprisesmechanically shaking said PTFE membrane filter material.
 24. A methodaccording to claim 23 wherein said collecting further comprises cleaningenergy from at least one sonic horn.